GRSM_GIS
Assuming that the users have already been added to your portal/organization, the following should get you started.
from arcgis import *
import csv
gis = GIS('portal url', 'admin', 'pass')
group = gis.groups.search('title: group title', '')
group[0].get_members()
userAdd = []
with open(r"path to csv") as f_input:
csv_input = csv.DictReader(f_input)
for row in csv_input:
users = row["provide column containing usernames"]
userAdd.append(users)
print(userAdd)
group[0].add_users(userAdd)
group[0].get_members()